error message "A pin name is expected" when testing the ALU

error message "A pin name is expected" when testing the ALU

I've built my ALU, but whenever I try to load it into the Hardware Simulator I get the error message:
"In HDL file C:\nand2tetris\nand2tetris\projects\02\ALU.hdl, line 51, a pin name is expected."
line 51 is the first component of my ALU, a Mux:
Mux(a=x[0], b=zero, sel=zx, out=1x0);
To my understanding, the error message implies that this line is missing some component pin, but i can't figure out where I went wrong. The only thing I can think of is the b=zero component, since I did not define what "zero" is, but that should default the value to 0 (since it's an unconnected input pin). Ive also tried explicitly defining "zero" by adding this Xor:
Xor(a=x[0], b=x[0], out=zero);
but this changed nothing.
Here's my full ALU:
// This file is part of www.nand2tetris.org
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/02/ALU.hdl

Re: error message "A pin name is expected" when testing the ALU

Changing "zero" to "false" doesn't change the error message that I get.
I realize now how to do the ALU more easily with the 16-bit chips, but I wasn't sure of how to do the syntax correctly at first so I built it all out of 1-bit chips. I could probably do it over with 16-bit chips, but I wanted to see if this implementation worked first (which is where I ran into the error message).